Young red spheroidal galaxies in the Hubble Deep Fields: Evidence for a truncated initial mass function at similar to 2 M-circle dot and a constant space density to z similar to 2

The optical-IR images of the northern and southern Hubble Deep Fields are u sed to measure the spectral and density evolution of early-type galaxies. T he mean spectral energy distribution is found to evolve passively toward a mid-F star-dominated spectrum by z similar to 2, becoming more sharply peak ed around the 4000 Angstrom break. We demonstrate with realistic simulation s that hotter elliptical galaxies would be readily visible if evolution pro gressed blueward and brightward at z > 2, following a standard initial mass function (IMF). The color distributions are best fitted by a "red" IMF def icient above similar to 2 M-circle dot and with a spread of formation in th e range 1.5 < z(f) < 2.5. Traditional age dating is spurious in this contex t; a distant elliptical can be young but appear red, with an apparent age g reater than 3 Gyr independent of its formation redshift. Regarding density evolution, we demonstrate that the sharp decline in numbers claimed at z > 1 results from a selection bias against distant red galaxies in the optical , where the flux is too weak for morphological classification, but is remed ied with relatively modest IR exposures that reveal a roughly constant spac e density to z similar to 2, with 32 and 16 elliptical galaxies detected ab ove and below z = 1, respectively. We point out that the lack of high-mass star formation inferred here and the requirement of metals implicates cooli ng flows of preenriched gas in the creation of the stellar content of spher oidal galaxies. Deep-field X-ray images will be very helpful in examining t his possibility.